Mistress can't rouse fancy (9)
I believe the answer is:
courtesan
'mistress' is the definition.
(courtesan is a kind of mistress)
'can't rouse fancy' is the wordplay.
'fancy' is an anagram indicator.
'cant'+'rouse'='cantrouse'
'cantrouse' anagrammed gives 'COURTESAN'.
(Other definitions for courtesan that I've seen before include "lover" , "Fanny Hill, perhaps" , "Prostitute - mistress of high-ranking man" , "Kept woman" , "loose woman" .)